This invention relates to quadraphonic sound systems, and more particularly to a decoder by which quadraphonic playback from a four channel recording on a stereo disc is achieved with maximum similarity to the four channel original master.
Matrix quadraphonic sound systems presently available involve the folding of four discrete channels into two, by encoding each of the four channels into distinct phase and amplitude characteristics in the two channel medium. It is then unfolded into four by means of a decoder.
Such systems provided heretofore are deficient in certain important respects: In a basic matrix system the conventional encode-decode sequence will position a sound source close to its intended location, but only if the listener is centrally seated and does not require that the source be as sharply defined as the four channel original master. For example, if a left front signal is presented to an encoder, the decoder will normally reproduce the left front signal at its intended location. However, cross talk also will appear at half power in the left rear and right rear channels, causing a subjective broadening of the original point source towards the rear pair of loudspeakers. A right front source will also appear in the two rear channels and a single channel source in the rear will also appear in both front channels.
The cross talk is much more noticeable to the non-central listener and also to the listener who moves his head. Such cross talk thus must be eliminated if an illusion of discreteness or resolution is to be obtained.
The conventional means for reducing such cross talk is to add to the basic matrix a circuit to sense the position of the momentarily loudest sound and to connect this sensing circuit to devices which reduce the gain in the channels which are expected to bear cross talk. For example, if the sensing device determines that left front is loudest, it reduces the gain on the two rear channels on the outputs of the basic decoder to prevent the cross talk from entering the loudspeakers.
In this second type of decoder the sound sources appear much sharper and appear to stand out from the background, independent of reverberation, even if the original four channel master had a gret deal of reverberation mixed in. In this regard, reverberation may be defined as a series of weak but important echoes, perhaps more than 20db down in level from the direct sound.
However, this second type of decoder, by its nature, suppresses reverberation since the sensing unit does not respond these low levels of sound. For example, if an instrument is playing in left front, the weak echoes encoded in left back and right back will appear at the outputs of the basic decoder, but so also will a much greater amount of cross talk from left front. The sensing unit "sees" the left front energy and shuts down the left back and right back outputs, thereby suppressing the reverberations, as well as the cross talk.
As previously mentioned, a basic decoder, ahead of the suppressing circuit, has poor front-to-rear separation. Accordingly, the weak rear reverberation is transferred as cross talk to the front, where it is not suppressed, and hence is permitted to appear along with the left front instrument. Accordingly, the listener, instead of hearing the hall reverberations in the rear, as in the original master, will hear it in the front, mixed along with orchestra, as in conventional stereo. Alternatively, the reverberations will flutter around the room if loud instruments stimulate front and rear parts of the decoder.
The foregoing conventional decode system may be characterized as having fixed decoding coefficients and one variable output gain per channel.
A third decode system provided heretofore may be characterized as having one variable decoding coefficient per channel and one variable output gain per channel, the variable coefficient being effective for eliminating cross talk coming from points exactly at center front or center rear. For example, when a center front signal appears, the rear channels are blended together electrically instead of being turned off. This makes them cancel a point source at center front only, and they pass all other non-cancelling sounds including rear reverberations. For sounds appearing in the corners, all reverberation is suppressed, as before.
FIG. 8 shows a Scheiber sphere illustrating the complex code used with the foregoing three examples of prior types of decoders.
A fourth decode system provided heretofore, but using a substantially different code, may be effectively characterized as having one variable decoding coefficient per channel. It can be made to alter the decoding characteristics themselves, rather than utilizing fixed decoding with variable gain. Thus, this type alters the decoding coefficients of the channels which are known to bear cros talk, to become identical with the one coefficient which cancels the source. For example, with this code, cross talk stemming from a left front source will appear at right front and left back, i.e. adjacent the source. Right front and left back then have their decoding coefficients altered, becoming the same as the silent channel which is this case, is right back. This cancels the crosstalk electrically, thereby eliminating it.
For the listener, this decoder system is effective over a full 360.degree. circle in suppressing unwanted cross talk and retaining reverberation. However, it is limited to codes which are "regular ", i.e., which cut through a Scheiber sphere on a plane, as illustrated in FIG. 7. Thus, since it cannot be utilized with other conventional encoders, it is limited in its use with but one type of encoded disc.
A distinction can be drawn between the code of the first three examples and the code of the fourth example. The fourth code may be described as "regular" or simple, in that a signal encoded to travel in a circle around the listener on play back is plotted as a great circle on a Scheiber sphere, with the plotting lying on a plane cutting through the diameter of the sphere. This is shown in FIG. 9. A complex code is one whose plotting deviates from a plane when the signal is encoded to travel around the listener on playback. This is illustrated in FIG. 8. These are two examples that are shown to fall into two broad classes of complex and simple codes, with many other variations being used or contemplated in each class.